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Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

This small study, examining historical bacterial DNA from various sources, demonstrated the existence of bacterial antibiotic-resistance genes long before the antibiotic era.

While these results are intriguing, independent confirmation will be necessary to fully rule out issues such as contamination of samples.

BOSTON -- What do pre-Columbian Incas, Renaissance-era Italian nobility, modern Italians, and indigenous tribes in South America have in common?

Intestinal bacteria that confer resistance to such antibiotics as vancomycin and penicillin, researchers reported here.

Remarkably, mummified Incan remains show the most resistant strains of bacteria, said Tasha Santiago-Rodriguez, PhD, a post-doctoral researcher at California Polytechnic University, San Luis Obispo. Those bacterial strains are resistant to antibiotics that weren't discovered for more than 500 years after the people died.

"The Incan mummies from the 10th to 14th centuries exhibited the highest overall number of putative antibiotic-resistance genes, suggesting that ancient vegetarian diets may be associated with the consumption of food contaminated with soil microbes harboring antibiotic-resistant genes," Santiago-Rodriguez told MedPage Today at her poster presentation at ASM Microbe 2016.

"I have to say we were surprised at the results of our study at first, but then when you think about how many of our antibiotics comes from nature, it makes sense." she said. "We did not expect to see such similar resistant genes in the guts of ancient individual when compared with modern-day individuals. In most of the cases of the ancient people we saw that there was a high representation of genes that are very similar to what we see today."

She said that the laboratory technicians double checked their work to make sure that there was no laboratory contamination that produced the unexpected findings. "We had to make sure. You have to think about that possibility," she said.

Santiago-Rodriguez and colleagues examined gut DNA from three Peruvian adult mummified remains; from five guts of Italian nobility from the 15th to the 18th centuries who also underwent mummification; from two modern day Italians and from two member of tribes living in the Amazon basin. The researchers were expecting to see bacterial resistance to multiple drugs among the contemporary Italians, but not in the ancient subjects nor in the Amazon tribes who do not access to antibiotics.

Santiago-Rodriguez said that the ancient DNA had degraded some, but was still good enough to perform analyses.

The researchers found resistance genes for vancomycin, multi-drug transporters, penicillin, macrolides, and many other antibiotics in all the guts of the living and dead subjects.

"We hypothesized that ancient dietary habits that consist mainly of crops and a greater environmental exposure may increase the reservoir of putative antibiotic-resistant genes in the human gut. Some of these antibiotics such as vancomycin are produced naturally in soil so it is possible that when these substances with antimicrobial properties were ingested in ancient times they established a molecular cascade the caused resistance genes to proliferate," Santiago-Rodriguez suggested. "That is what we think happened."

The researchers applied targeted and metagenomics to identify sequences homologous to antibiotic-resistance in the mummified gut remains.

While the resistance genes identified in the study are close in characteristics to modern-day antibiotic genes, the researchers are still not positive that when they have discovered will truly be antibiotic resistant. "We are now doing those experiments," she said.

Santiago-Rodriguez said that the study informs "that antibiotic resistance is ancient and it may be a good way to try to see if we develop good models how antibiotic resistance is going to evolve in the future. If we know more about how these genes evolve we may be able to use that information to crease better and more efficient antibiotics."

"I think it is a reasonable explanation of why these ancient people would have resistant genes. Vancomycin comes from soil so that is a possibility," said Brandon Eilertson, MD, of the State University of New York Downstate Medical Center in Brooklyn. "But I wonder about the high-status nobility who clearly would not have been working in the fields and how they developed the resistance genes.

"These are very interesting findings, but the sample size is small," he noted.

Santiago-Rodriguez and Eilertson disclosed no relevant relationships with industry.

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